Reluctance machines are well known in the art. In general a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move to a position where the inductance of an excited winding is maximized (i.e. the reluctance is minimized).
In one type of reluctance machine the phase windings are energized at a controlled frequency. This type of reluctance machine is generally referred to as a synchronous reluctance machine. In another type of reluctance machine, circuitry is provided to determine the position of the machine's rotor, and the windings of a phase are energized as a function of rotor position. This type of reluctance machine is generally referred to as a switched reluctance machine. Although the description of the current invention is in the context of a switched reluctance machine, the present invention is applicable to all forms of reluctance machines, including synchronous and switched reluctance motors and to other machines that have phase winding arrangements similar to those of switched reluctance machines.
The general theory of design and operation of switched reluctance machines is well known and discussed, for example in The Characteristics, Design and Applications of Switched Reluctance Motors and Drives, by Stephenson and Blake and presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
As a switched reluctance motor (or generator) operates, magnetic flux is continuously increasing and decreasing in different parts of the machine. The changing flux results in fluctuating magnetic forces being applied to the ferromagnetic parts of the machine. These forces can produce unwanted noise and vibration. One major mechanism by which these forces can create noise is the ovalizing of the stator caused by magnetic forces normal to the air-gap. Generally, as the magnetic flux increases along a given diameter of the stator, the stator is pulled into an oval shape by the magnetic forces. As the magnetic flux decreases, the stator pulls or springs back to its undistorted shape. This ovalizing and springing back of the stator will produce audible noise and can cause unwanted vibration.
In addition to the stator distortions resulting from the phenomena described above, the fluctuating magnetic forces in the motor can distort the stator in other ways, as well as distorting the rotor and other parts of the machine system. For example, distortions of the rotor can cause resonance of the rotor end-shields. These additional distortions are another potential source of unwanted vibration and noise.
Another source of undesirable noise in reluctance machines include the "siren" effect often associated with such machine. As explained above, the rotors and stators used in most reluctance machines include salient poles. The mechanical structure resulting from the placement of such a salient pole rotor within a core defined by a salient pole stator is similar in some respects to a siren and can, in some instances, produce unwanted audible noise according to the same phenomenon taken advantage by a siren to produce noise.
A still further source of unwanted noise associated with some reluctance machines is the noise that results as air is drawn into and passed through the reluctance machine during operation. During operation of many reluctance machines, the rotation of the rotor, especially at high speeds, tends to draw air into and through the cavity within which the rotor is positioned resulting in undesirable "windage" noise.
Although the problem of unwanted acoustic noise and vibration has been recognized, known control systems for reluctance motors do not adequately solve the problem. One proposed solution has been to implement complicated control techniques to carefully control the currents applied to the phase windings of the machines. Certain of such approaches are generally discussed in C. Y. Wu and C. Pollock, "Analysis and Reduction of Vibration and Acoustic Noise in the Switched Reluctance Drive," Proceedings of the IAS '93 pp. 106-113 (1993). While these current control approaches can reduce the noise produced by a reluctance machine they are often difficult and costly to implement. Moreover, these types of reduced-noise current switching schemes constitute only one possible form of noise reduction and do not necessarily remove all of the sources of potentially unwanted noise.
It is an object of the present invention to provide for a reduced noise reluctance machine that can be easily constructed and that can operate, in isolation or in combination with reduced-noise current switching schemes to reduce the noise produced by a reluctance machine.